In recent years, with miniaturization of design rules, a film thickness distribution of an SOI layer of an SOI wafer for use in fabrication of an SOI device, especially fabrication of an FD-SOI (Fully Depleted SOI) device has affected a device manufacturing process as well as transistor characteristics. In an integrated circuit, uniforming characteristics of transistors constituting a circuit is important.
As an existing film thickness measuring method for calculating a film thickness distribution of a thin film of a thin-film-formed wafer that has the thin film on a surface of the substrate, film thickness measurement for each point based on spectroscopic ellipsometry or reflection spectroscopy is general, but a film thickness distribution measuring device that can perform film thickness distribution measurement in a wide plane with a resolution of approximately 1 μm is not commercially available.
In the point measurement based on the spectroscopic ellipsometry or the reflection spectroscopy, a spectrum in a given wavelength range (which is generally a visual light range) is taken in accordance with each measurement point, and this spectrum is fitted into a model film architecture, whereby film thickness at each measurement point is obtained. Therefore, when performing measurement with a resolution of approximately 1 μm is tried, the number of the measurement points is extremely increased, and hence the measurement is actually impossible because of the limitation in amount of calculation and time.
Further, to effect the spectral measurement, since a wavelength region in a broad wavelength range is required, it is practically impossible to increase a spatial resolution and effect the multipoint film thickness measurement. Therefore, as a device that can collectively measure the entire surface of the wafer by using these methods, there is only a device with a spatial resolution of approximately several 100 μm under existing conditions.
As described above, there has been demanded the method for evaluating a thin-film-formed wafer, which can highly accurately and simply perform the measurement of a film thickness distribution of a thin film, especially an SOI layer on an entire surface of the wafer at a low cost.
It is to be noted that Patent Document 1 discloses a technology of applying white light to an SOI, dispersing reflected light in accordance with each wavelength, and calculating an SOI layer film thickness from interference information of each wavelength, and Patent Document 2 discloses that a laser beam of 488 nm is applied to an SOI layer which is less than 1 μm, regular reflected light is detected, and a variation in film thickness within the plane is inspected based on an interference pattern formed with irradiation light.